1. Field of the Invention
The present invention relates to a nonaqueous electrolyte secondary battery such as a lithium ion secondary battery.
2. Description of the Related Art
The nonaqueous electrolyte secondary battery is charged and discharged by the migration of lithium ions between the negative electrode and the positive electrode. Vigorous research is being conducted on the nonaqueous electrolyte secondary battery as a battery having a high energy density.
In recent years, various characteristics are required for the nonaqueous electrolyte secondary battery. For example, a nonaqueous electrolyte secondary battery used as a power source for a digital camera is expected to be used under the discharge of about 3 C. Also, a nonaqueous electrolyte secondary battery for a vehicle such as a hybrid electric motor car is expected to be used under the discharge not lower than about 10 C. Such being the situation, high rate characteristics are required for the nonaqueous electrolyte secondary battery. Further, since the nonaqueous electrolyte secondary battery for a vehicle is expected to be used under a high temperature environment, an improvement in the high temperature characteristics is also required for the nonaqueous electrolyte secondary battery.
A nonaqueous electrolyte secondary battery using a lithium titanium oxide as the negative electrode active material has already been put to a practical use because the secondary battery is small in its expansion and shrinkage accompanying the charge-discharge of the secondary battery and is excellent in that the secondary battery has a long life. However, the nonaqueous electrolyte secondary battery using the lithium titanium oxide is defective in that the lithium titanium oxide is poor in its electrical conductivity and that the battery is inferior in the high rate characteristics.
Under the circumstances, it is attempted to improve the high rate characteristics by using a carbonaceous material such as acetylene black or carbon black as the conductive agent. However, the demands for the high rate characteristics are particularly strong. If the mixing ratio of the conductive agent is increased for meeting the particular demands, the energy density of the negative electrode is lowered.
A measure for improving the high rate characteristics is proposed in Japanese Patent Disclosure (Kokai) No. 2003-163029. Specifically, it is proposed to use a nonaqueous electrolyte containing as an additive ethylene sulfite represented by chemical formula (1) given below or propane sultone represented by chemical formula (2) given below:

According to the patent document quoted above, the low electrical conductivity of lithium titanate is derived from the rutile type titanium dioxide phase formed on the surface of lithium titanate. It is taught that ethylene sulfite or propane sultone forms an organic film of a high ionic conductivity in the initial charging stage of the secondary battery so as to cover the rutile type titanium dioxide phase having a low conductivity, with the result that the high rate characteristics of the secondary battery can be improved. However, this measure is incapable of achieving the high rate characteristics sufficiently.